1. Field of the Invention
The present invention relates to a data slicer, and more particularly, to a data slicer which effectively compensates for wobble and asymmetrical phenomena due to optical and electrical characteristics of a disk.
2. Description of the Related Art
In a disk such as a DVD or a DVD-RAM, information is recorded in sector units. Each sector is comprised of a header information area having a physical identification data (PID) area and a user data area. FIGS. 1A and 1B show the format of header information recorded on a DVD-RAM disk. FIG. 1A shows the header information recorded at the first sector of a track and FIG. 1B shows the header information recorded at the other sectors.
As shown in FIGS. 1A and 1B, reference numerals 10 and 12 represent respectively a header information area and a user data area where user data is recorded. The header information area 10 is divided into a peak header area 10a and a bottom header area 10b, and the user data area 12 is divided into a land area and a groove area.
In a DVD-RAM disk, a track is formed spirally and the track is shifted laterally at a predetermined reference point. The reference point usually becomes a starting point of the first sector.
Referring to FIG. 1A, the lateral shift of the track occurs at the first sector. The header information at the first sector is disposed such that the peak header 10a occurs prior to the bottom header 10b when the next track is a land, whereas the header information at the first sector is disposed such that the bottom header 10b occurs prior to the peak header 10a when the next track is a groove. Thus, it can be identified whether the next track is a land or a groove.
Wobble is present in the user data area 12. The wobble means a wave formed between the land and the groove, as shown in FIGS. 1A and 1B, that is, corrugation of the side walls of a track (land or groove).
When a pickup device (not shown) reads signals recorded on tracks, a low frequency signal corresponding to the frequency of the wobble is superposed on an RF signal. The low frequency signal is referred to as a wobble signal and the wobble signal has a predetermined frequency according to the DVD specification.
As shown in FIGS. 1A and 1B, it can be seen that no wobble is formed in the header information area 10. This is because the header information is divided into the peak header 10a and the bottom header 10b and the peak header and the bottom header are recorded across the boundary of tracks. Thus, to guarantee recording of data in a correct position, an area such as buffer, gap or guard is inserted between the header information area 10 and the user data area 12.
FIG. 2 is a waveform diagram showing the relationship between a header signal and an RF signal. As shown in FIG. 2, the RF signal is comprised of a peak header portion 20, a bottom header portion 22, and a user data portion 24. The peak header portion 20 has a similar peak-peak value and a greater envelope than the user data portion 24 while the bottom header portion 22 has a similar peak-peak-value and a smaller envelope than the user data portion 24.
In the RF signal, since an asymmetrical phenomenon is generated due to the header signal, a level, i.e., a slice level, for slicing the data according to the change of the RF signal is necessary to accurately reproduce the original information.
FIG. 3 is a waveform diagram shown the relationship between the wobble signal and the RF signal. In FIG. 3, the wobble signal corresponds to a frequency component taken by connecting the middle points of the RF signal. As shown in FIG. 3, the RF signal is indicated as being superposed on the wobble signal and a level which slices data according to change of the wobble signal is necessary.
FIG. 4 is a block diagram showing the configuration of a conventional data slicer. Referring to the drawing, the data slicer includes a comparator 40, a low pass filter (LPF) 42, and an amplifier 44. In the apparatus shown in FIG. 4, a loop is applied through the amplifier 44 to trace such that the average electrical power of a sliced data pulse signal output from the comparator 40 is equal to a reference voltage V.sub.REF. Thus, the comparator 40 finally outputs a signal having a duty ratio of 50%.
However, the apparatus shown in FIG. 4 cannot accurately trace the center level of the RF signal which becomes a slice level when an input signal changes rapidly or the wobble signal becomes greater. That is, although low-pass filtering should be applied strongly to trace the average electric power, since the low pass filter 42 is characteristically stable, an input wobble signal over a set frequency cannot be followed. Particularly, the above problem becomes serious as the DVD-RAM has a lots of signals, such as header signals, which change rapidly.